The Tentacles from Below

Six thousand feet below the service of the ocean, with two thousand more to go, Commander Keith Wells and the crew are working to chart the Newfoundland banks. Cataclysms had begun to tear up the sea-floor of this region, seismographs and cable companies had reported titanic upheavals and sinkings of the ocean bed, changing hundreds of miles of underwater territory.
This was to be a routine mission for the NX-1, a super-submarine fresh from the yards, small, but modern to the last degree and the mother-ship Falcon. When a nearby fishing fleet disappears in calm seas the routine mission turns into a mission of survival.
Down to tremendous ocean depths goes Commander Keith Wells in his blind duel with marauding "machine-fish".